1. Field of the Invention
The present invention generally relates to an image data compressing technique and, more particularly, to an image processing system which compresses and expands image data by using a sub-band encoding method.
The image processing system related to the present invention may be used in a digital copy machine, a facsimile machine, a digital printer, a digital camera or a digital video camera, and also may be used in an image recording system such as a CD-ROM drive or a floppy disc drive.
2. Description of the Related Art
A sub-band encoding method such as the discrete cosine transform (DCT) or the Harr Wavelet transform is considered a method for effectively compressing a continuous tone image. Additionally, Japanese Laid-Open Patent Application No. 2-305272 discloses another method for encoding image data by separating an image area into a character area and a halftone area so as to encode these areas by an encoding method appropriate for each of the areas.
Such a method for compressing image data using the sub-band transform such as the DCT or the Harr Wavelet transform can effectively compress a continuous tone image. However, there is a problem in that a compression rate is low when a complete binary image is compressed.
Additionally, in a digital copy machine, even if an original image is a complete binary image, image data obtained by scanning such a complete binary image may become incomplete binary image data due to fluctuation in a scanning operation. Thus, there may be a problem in compressing such incomplete binary data by an entropy-encoding method due to fluctuation in a scanned image.
As a method for rotating or sorting images in a copy machine, a block truncation encoding (BTC) which is one of fixed length encoding methods is popular. However, there is a problem in that a compression rate for an entropy encoding is low as compared to that of a sub-band transform method, and a calculation is complex.
In an image forming apparatus such as a copy machine or a printer, image data obtained by a scanner is subjected to gamma correction or a filtering process so as to adjust image quality. The thus-processed image data is stored in a memory, and then the image data is sent to a printing unit.
Generally, such image data is subjected to a data compression in order to reduce a capacity of the memory that stores the processed image data. Generally, in a data compressing method, image data is transformed into frequency components by using an orthogonal transformation such as the discrete cosine transform (DCT), and the quantized image data is subjected to an entropy encoding. Dispersion of a high-frequency factor in the frequency transformation factors varies in response to a magnitude of change in intensity of the image. Thus, the image quality is improved when a quantizing method is changed in response to a type of an area to be processed.
Japanese Laid-Open Patent Application No. 7-74959 discloses a technique in which a quantization table is changed based on a transform factor obtained by an orthogonal transformation of an original image by each individual block so that the image quality matches the contents of the image data and a compression rate is improved.
When an image is printed by a copy machine, a character image and a line image can be well recognized by rendering the intensity slope of a contour of the characters or the lines to be steep. On the other hand, when an image having a gentle intensity slope such as a photograph is printed, a random change in the intensity having a small amplitude is sensed as a noise. Thus, it is preferred for such a photographic image to reduce the intensity slope of an output image. Particularly, in a mesh point photographic image, a better image quality can be obtained by reducing the intensity slope even for an area having a steep intensity slope.
Accordingly, an edge area corresponding to a character image or a line image is separated from a mesh point image and a gentle slope area of a photographic image so that the edge area is subjected to a differential filtering process whereas the photographic image is subjected to a smoothing filtering process. Additionally, when an image data compression is performed, another separation of image areas is performed in response to degrees of the intensity slope in edge areas.
As mentioned above, in the conventional technique, two separation processes are performed on the same image data and the filtering process is performed separately from the quantizing process. Thus, there is a problem in that a process time is increased and a hardware cost is increased. Additionally, there is a disadvantage in the technique disclosed in the above-mentioned patent document in that a compression rate is not minimized since a result of the area separation must be also stored as the compressed data.
An image data compression technique is generally used in the image data processing field so as to reduce a capacity of a memory for storing image data or reduce a time for transmitting image data. There are various image data compressing methods depending on the processing modes of image data. When image data is printed, a rotation of the image may be requested. In order to rotate the image at a high speed, a fixed length compression is used.
Additionally, when image data is exchanged between systems having different resolutions or gradation characteristics, a compressing method using a layered data structure is desired so as to select transmission data corresponding to an image quality of an image outputting system. Especially, when image data is transmitted to a display apparatus, a progressive transmission method is required. In the progressive transmission method, image data of an object such as an icon can be transmitted prior to sending the image data. Thus, data compression is performed in response to the level of layers.
Additionally, when a trial printing is performed for checking a layout while reducing toner consumption in an image printing apparatus, a data compressing method is required by which a feature of the image is maintained but image quality is not reduced.
Japanese Laid-Open Patent Application No. 1-135265 discloses a data compressing method in which an original image is divided into a plurality of blocks, and each block is divided into image data which is orthogonal-transformed and other data so that a representative image of the image file can be effectively regenerated.
Generally, an image comprises an image area and an edge area. In the image area, a gradation of the image gradually changes, such as in a photograph or a graphic image. In the edge area, a gradation sharply changes in an area of an edge of the image and an area adjacent to the edge, such as in a character image or a line image. When the visual sense of human beings is considered, gradation is important in the image area whereas resolution is important in the edge area.
In the conventional technique disclosed in the above-mentioned patent document, a sampling is performed on the original image, and the sampled image data is subjected to the discrete cosine transform (DCT). The same transform factor which is obtained from a quantization table is used for all areas. In such a case, a length of data is fixed since a single quantization table is used. However, there is a drawback in that the quantized image data does not accurately represent the feature of the image while a large amount of data is used since a single quantization is used. Additionally, there is a disadvantage in that only two levels of image data can be selected.